This invention relates to switching dc-to-dc converters, and more particularly to such converters requiring large step-down or step-up conversion ratios without a transformer.
There is a growing need for switching dc-to-dc converters having large conversion ratios, for example, a converter/regulator to provide a +5 V output from a nominal -48 V input. Such a converter should have a low profile capable of assembly on a plug-in card for insertion in a conventional rack. Consequently, there is an obvious motivation to use a switching converter operating at a frequency of at least hundreds of kHz, in order to reduce the size of magnetic cores and to take advantage of hybrid construction techniques.
With the advent of power MOSFET transistors, the switching frequency is limited by the magnetic cores rather than by the switch. More specifically, it is a transformer that limits the frequency, rather than an inductor, because the ratio of leakage to primary inductance increases in a transformer as the physical size decreases. A transformer commonly provides two functions in a dc-to-dc switching converter: it provides dc isolation, and it provides an additional voltage conversion ratio over and above that available from the switch duty ratio.
In applications where dc isolation is not needed, a transformer (or an autotransformer) would normally still be required if the necessary voltage conversion ratio is large. For example, in a 50 V-to-5 V converter, the switch duty ratio would have to be about 0.1 if a transformer were not used, which severely limits the switching frequency and the dynamic range of the converter, and also has undesirable implications with respect to peak currents, loss of efficiency, and noise. On the other hand, if a transformer were used, the switching frequency would also be severely limited by the transformer itself.